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Lecture-3_

1.

Digital Logic Design
Lecture – 3:
Combining Logic Gates
Konakbayev Olzhas, senior-lecturer,

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Lecture base
Digital Electronics: Principles & Applications, 9th edition by Roger
Tokheim & Patrick E. Hoppe:
• Chapter 4
2

Topics to cover
• Constructing Circuits from Boolean Expressions
• Minterm and Maxterm Boolean Expressions
• Boolean Expression from a Truth Table
• Truth Tables and Boolean Expressions
• Simplifying Boolean Expressions
• Karnaugh Maps
• Using NAND Logic
• Using DeMorgan’s Theorem
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4.

Circuits from Boolean Expressions
1a
• AND-OR pattern of gates from a Sum-of-Products
Boolean expression.
A B A C B C Y
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Circuits from Boolean Expressions
2
• OR-AND pattern of gates from a Product-of-Sums
Boolean expression.
(A B) (A B C) Y
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Circuits from Boolean Expressions
3
• Step 1: Identify pattern of operation
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Circuits from Boolean Expressions
4
• Step 2: Add the first OR gate with inputs
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Circuits from Boolean Expressions
5
• Step 3: Add the second OR gate with inputs
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9.

Minterm and Maxterm Boolean Expressions
1
• Maxterm Boolean Expression
C+B+A C+B+A C+B+A =Y
(Product-of-Sum Expression)
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Minterm and Maxterm Boolean Expressions
2
• Minterm Boolean Expression
C B A C B A C B A Y
(Sum-of-Product Expression)
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11.

Boolean Expression from a Truth Table
• Step 1: Write an AND
expression for each
instance Y = 1.
• Step 2: OR the
expressions together
to form a Boolean
expression.
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12.

Truth Tables and Boolean Expressions
Let’s look at an example.
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Simplifying Boolean Expressions
1
• Original Boolean Expression
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Simplifying Boolean Expressions
2
• Reduced Boolean Expression
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Single-variable Boolean Theorems
1
• The first Boolean theorem states that X ANDed with
0 is 0.
X 0 0
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Single-variable Boolean Theorems
2
• The second Boolean theorem states that X ANDed
with 1 is X.
X 1 X
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Single-variable Boolean Theorems
3
• The third Boolean theorem states that X ANDed with
X is X.
X X X
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Single-variable Boolean Theorems
4
• The fourth Boolean theorem states that X ANDed
with ഥ
X is 0.
X X 0
X
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19.

Single-variable Boolean Theorems
5
The fifth Boolean theorem states that X ORed with 0 is X.
X+0=0
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Single-variable Boolean Theorems
6
• The sixth Boolean theorem states that X ORed with 1
is 1.
X+1=1
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Single-variable Boolean Theorems
7
• The seventh Boolean theorem states that X ORed
with X is X.
X+X=X
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Single-variable Boolean Theorems
8
• The eighth Boolean theorem states that X ORed with
ഥ
X is 1
X X 1
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Single-Variable Boolean Theorems
9
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Multivariable Boolean Theorems
1
The ninth Boolean theorem states that X ORed with Y is
equal to Y ORed with X.
X+Y=Y+X
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Multivariable Boolean Theorems
2
• The tenth Boolean theorem states that X ANDed with
Y is equal to Y ANDed with X.
X Y Y X
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Multivariable Boolean Theorems
3
• The eleventh Boolean theorem demonstrates when
ORing multiple inputs, the order of operation does
not matter.
X Y Z X Y Z X Y Z
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Multivariable Boolean Theorems
4
• The twelfth Boolean theorem demonstrates when
ANDing multiple inputs, the order of operation does
not matter.
X Y Z X Y Z X Y Z
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Multivariable Boolean Theorems
5
• The thirteenth Boolean theorem is sometimes called
the distributive theorem.
X Y Z X Y X Z
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Multivariable Boolean Theorems
6
• The fourteenth Boolean theorem states X ORed with
X ANDed with Y is equal to X.
X X Y X
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Multivariable Boolean Theorems
7
• The fifteenth Boolean theorem states X ORed with ഥ
X
ANDed with Y is equal to X ORed with Y.
X X Y X Y
Access the text alternative for slide images.
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Multivariable Boolean Theorems
8
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Multivariable Boolean Theorems
9
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Boolean Reduction with Two Variables
1
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Boolean Reduction with Two Variables
2
• Let’s check our answer.
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35.

Boolean Reduction with Three Variables
1
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Boolean Reduction with Three Variables
2
• Let’s check our answer.
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37.

Karnaugh Maps
• The columns and rows of the two and three input
Karnaugh maps must be laid out in this manner.
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38.

Karnaugh Maps with Three Variables
1
Step 1: Write the minterm for each input
combination that produces a 1 on the
output.
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39.

Karnaugh Maps with Three Variables
2
Step 2: Write the Sum of Product expression
using the minterms.
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Karnaugh Maps with Three Variables
3
Step 3: Fill in the 1’s for each minterm in
The Sum of Product expression
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Karnaugh Maps with Three Variables
4
Step 4: Loop adjacent 1’s in groups of
two, four, or eight.
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Karnaugh Maps with Three Variables
5
Step 5: Simplify by dropping terms that
contain a term and its complement within a
loop.
Step 6: OR the remaining terms together to
form a simplified Boolean expression.
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43.

Karnaugh Maps with Three Variables
6
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Boolean Reduction with Three Variables
• Let’s check our answer.
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Karnaugh Maps with Four Variables
1
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Karnaugh Maps with Four Variables
2
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Other Looping Possibilities
1
• Consider the Karnaugh map as a vertical cylinder.
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Other Looping Possibilities
2
• Consider the Karnaugh map as a horizontal cylinder.
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Other Looping Possibilities
3
• Consider the Karnaugh map as a ball
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Using NAND Logic
1
• Start with a minterm (sum-of-products) Boolean
expression.
• Draw the AND-OR logic diagram using AND, OR, and
NOT symbols.
• Substitute NAND symbols for each AND and OR
symbol, keeping all connections the same.
• Substitute NAND symbols with all inputs tied
together for each inverter.
• Test the logic circuit containing all NAND gates to
determine if generates the proper truth table.
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